I get the above 'fatal error' when trying to load the DBG extension. Is this a known problem? Should I try an earlier version of DBG? If so, any idea which one? Will DBG work with another version of Turck?

There's no mention of this problem on either site (forums or buglists). I'm mailing mmcache@turckware.ru with this too, hopefully there's an easy solution...

Since the MMTurck extension is an optimiser and you probably don't require it in your development environment (where you want the debugger) just comment out the whole section in the php.ini file.
It certainly gets rid of error and so far has had no other repercussions.

I am not getting the same problem as you guys are, on a fairly similar setup. The only weird thing that is happening for me is that both of the modules show up in my phpinfo, but there is no DBG section.

it depends on the way you load the extensions. If DBG fails to initialize as a php extension it just can not show the section.
Show us your settings in php.ini, including dbg.so (or php_dbg.dll) itself.